Japahn once remarked that Languish is actually better than
Damnation in some situations since you can abuse the symmetry with a big creature on your side. In your cube, it looks like the black creatures that can abuse that would include
Carrion Feeder,
Fallen Angel,
Nightmare,
Gurmag Angler, and some others in occasionals or dipping into the other colors. In general I don't see a lot of 5 toughness or effects that increase toughness. If I was drafting your list, I think I'd consider Languish to be nearly on par with Damnation, and it seems really appealing to put out one of the sacrifice creatures (Carrion Feeder / Fallen Angel) and then sacrifice stuff like
Virus Beetle and
Bone Shredder to keep it alive through a Languish. That would rock. I would expect Languish to kill almost any creature my opponent makes unless they're green, and I'd draft spot removal to mop that up.
I can't add any experience with Earthquake / Black Sun's Zenith, but I have both Day of Judgment and Languish. I think Languish is a strong pick in my cube. I have some stuff that is or can be more than 4 toughness, so it's not as strong, but that presents opportunities for fun abuse of the card as well. I think that's what you'd want, because otherwise there isn't much point in including Languish over Damnation.
None of this is new or surprising to you, but my main point is that Languish is a strong and appealing card to open.
Edit: Black Sun's Zenith is such an interesting comparison. In your CCC cube, when I filter the core cards of your cube, a big majority have toughness 2 or less, which makes these two spells very similar in your context. If you're specifically looking at casting it on turn 4, most creatures just die either way, and adding two or three -1/-1 counters effectively nerfs most of the creatures that don't die to Zenith but which would die to Languish. How long do your games go? How common is it for a player to get 6-7 lands in play? The Zenith can be better in those situations or if you can use its higher level of agency and flexibility to your advantage; the trouble is, even if you wipe the opposing board, you end up with shrunken creatures on your side of the board. The flexibility of the Zenith (and Mutilate, which you have in occasionals) is cool for agency. But I almost feel like this acts like a finisher choice, where either choice does approximately the same thing.